Such a power supply control system is provided, for example, in an audio recording and/or reproducing installation including signal sources, e.g. a tape deck and a tuner, and a main or power amplifier for amplifying both signals from the signal sources. In this system, a relay is provided which is energized by closing the power supply switch of the tape deck or the tuner thereby to connect through its make contact the main amplifier to the power supply. The relay is necessarily maintained energized when the installation works, with the result that the necessary relay-energizing current through the power transmission lines has an undesirable influence on the regulation of the installation. Moreover, in the case of such an installation mounted on a vehicle e.g. a motorcar, the reliability of the relay falls and chatter of the relay contact tends to occur due to inevitable vibrations of the vehicle; as a result of which the installation tends to erroneously operate or get out of order.
To avoid such undesirable results attendant on the use of a relay for connecting the main amplifier to the power supply, power supply control systems having electronic switching devices such as transistors in place of the relay have been proposed, but none of them is successful in obtaining the desired reliability.
A power supply control system of an installation having signal sources, which comprises a tuner and a tape deck, and a main amplifier for amplifying both the signals from the signal sources is known, in which electric power is fed to a selected one of the signal sources while the signal from the energized one of the signal sources alone is transmitted to the main amplifier.
With respect to power supply switches, the known power supply system is arranged such that the signal sources only are provided with manually operated power supply switches and power supply to the main amplifier is controlled by operating the power supply switches of the signal sources. In this known system, no switch means is provided for selectively transmitting the signals from the signal sources to the main amplifier for the purposes of reduction in number of manually operated switches, improvement in reliability and reduction in size of the installation.
This known system has a drawback that both signals from the two signal sources can be simultaneously transmitted to the main amplifier and amplified thereby when both the power supply switches of the two signal sources are rendered conductive by accidental manual operations.